In a continuous steam generator the heating up a number of evaporator tubes which together form this gas-tight surrounding wall of a combustion chamber leads to a complete evaporation of the flow medium in the evaporation tubes in one pass. The flow medium—usually water—is fed after its evaporation to the superheater tubes connected downstream of the evaporator tubes and is superheated there. The position of the evaporation end point, i.e. the boundary area between unevaporated and evaporated flow medium, is variable and dependent on operating mode in this case. In full-load operation of this type of continuous steam generator the evaporation end point lies for example in an end area of the evaporation tubes so that the superheating of the evaporated flow medium already begins in the evaporator tubes. By contrast with a natural or forced-circulation steam generator, a continuous steam generator is not subject to any pressure limitation, so that it can be designed for fresh steam pressures far above the critical pressure of water (Pcri≈221 bar)—where no distinction of the phases water and steam and thus no phase separation either is possible.
In off-peak operation or during start-up this type of continuous steam generator is usually operated with a minimum flow of flow medium in the evaporator tubes in order to guarantee a safe cooling of the evaporator tubes. To this end, even with low loads of for example less than 40%, the design load of the pure mass throughflow through the evaporation is generally no longer sufficient for cooling the evaporator tubes, so that the throughflow of flow medium circulated through the evaporator is overlaid with an additional throughflow of flow medium. The operational minimum flow of flow medium provided in the evaporator tubes is thus not completely evaporated in the evaporator tubes during start-up or in off-peak operation, so that with this type of operating mode there is still unevaporated flow medium, especially a mixture of water and steam, present at the end of the evaporator tubes.
Since the superheater tubes usually connected downstream of the evaporator tubes of the continuous steam generator only once the flow medium has passed through the walls of the combustion chamber are however not designed for a throughflow of unevaporated flow medium, continuous steam generators are usually designed so that, on start-up and in off-peak operation, entry of water into the superheater tubes is securely avoided. To this end the evaporator tubes are usually connected to the superheater tubes downstream from them via a water separator system. The water separator in this case effects a separation into water and steam of the water-steam mixture coming out of the evaporator tubes during start-up or off-peak operation. The steam is fed to the superheater tubes connected downstream from the water separator, whereas the separated water can for example be fed back into the evaporator tubes via a recirculation pump or discharged via a pressure relief device. A continuous steam generator of the design mentioned above is known for example from DE 197 02 133 A1.
With this type of continuous steam generator the evaporator tubes forming the lower part of the surrounding wall of the gas draught usually open out into one or more outlet collectors from which the flow medium is directed into a downstream water-steam separator. In this device the flow medium is separated into water and steam, with the steam being transferred into a distribution system connected upstream of the superheater tubes, where a distribution of the steam mass flow to the individual flow medium-side parallel-connected superheater tubes is undertaken.
In this type of construction the evaporation end point of the continuous steam generator is fixed by the intermediate connection of the water separator system in start-up and off-peak operation and not—as in full-load operation—variable. This means that the operational flexibility when the continuous steam generator is constructed in this way is significantly restricted in off-peak operation. Furthermore, with this type of construction, the separator systems must as a rule, especially as regards the choice of material, be designed so that the steam in the separator or in pure continuous mode is significantly overheated. The necessary choice of material also leads to a significant restriction in operational flexibility. As regards the dimensioning and type of construction of the components required, said construction also requires that the water escaping during start up of the continuous steam generator in a first start-up phase must be completely captured in the separator system and must be able to be discharged via the downstream separator vessel and the outlet valves into the pressure relief unit. The resulting comparatively large dimensioning of separator vessel and outlet valves leads to a significant outlay in manufacturing and installation.